This invention relates to a method and apparatus for fatigue testing, and more particularly to a method and apparatus for detection and analysis of crack initiation crack propagation and fracture detection.
The testing and analysis of materials and components in order to predict the life of such materials or components, or in order to detect imminent failure, has been the subject of considerable research and development over recent decades. Predicting and detecting the failure of materials or components in such critical applications as aircraft and aerospace equipment and medical equipment, particularly medical equipment implanted in the body, has been the subject of considerable research and development.
Fatigue testing of materials in the laboratory is commonly used to obtain data for predicting the life of components which may be fabricated from such material. It is also used to compare the ability of various materials to withstand fatigue. Such testing commonly utilizes a representative sample of a metal or alloy which is clamped in test fixture and subjected to repeated stresses with means provided to monitor the number of stress applications, and to detect the initiation and/or propagation of cracks, while other machines are frequently provided to detect the fracture or failure of a test specimen.
Data obtained from such testing is then used to compare the ability of different materials to withstand stresses, and to predict the potential life of components constructed of such materials. Methods used to detect changes in the test specimen include measuring the voltage across a portion of the test specimen, or resulting from current flow between points on the surface of the test specimen Also, it is known to provide notched sections in the test material of a standard or fixed configuration to provide preselected stressed areas within which the crack will form and propagate.
One other method commonly used in the analysis and fatigue prediction of components such as aerospace components is applying a resistive film to the surface of the component, commonly referred to as a strain gauge, and measuring changes in the resistance of the strain gauge due to stresses placed on the strain gauge by the surface of the component being tested.
All of the many and various prior art methods and apparatus of fatigue testing have not proved entirely satisfactory in providing desired test information and results. The correlation between laboratory testing and surface failure has often been poor and less than that desired.
Prior art devices have not provided the desired versatility in applying stresses to test specimens which more clearly relate to those encountered in actual use. In addition, it is desirable to include in a single versatile fatigue analysis apparatus and method the ability to detect the initiation of cracks and to detect and monitor the propagation of such cracks, along with the detection of a fracture or failure of the test specimen.